Reference: The Call of Cthulhu by H.P. Lovecraft
The bas-relief was a rough rectangle less than an inch thick and about five by six inches in area; obviously of modern origin. Its designs, however, were far from modern in atmosphere and suggestion; for, although the vagaries of cubism and futurism are many and wild, they do not often reproduce that cryptic regularity which lurks in prehistoric writing. And writing of some kind the bulk of these designs seemed certainly to be; though my memory, despite much the papers and collections of my uncle, failed in any way to identify this particular species, or even hint at its remotest affiliations.
Above these apparent hieroglyphics was a figure of evident pictorial intent, though its impressionistic execution forbade a very clear idea of its nature. It seemed to be a sort of monster, or symbol representing a monster, of a form which only a diseased fancy could conceive. If I say that my somewhat extravagant imagination yielded simultaneous pictures of an octopus, a dragon, and a human caricature, I shall not be unfaithful to the spirit of the thing. A pulpy, tentacled head surmounted a grotesque and scaly body with rudimentary wings; but it was the general outline of the whole which made it most shockingly frightful. Behind the figure was a vague suggestions of a Cyclopean architectural background.
They worshipped, so they said, the Great Old Ones who lived ages before there were any men, and who came to the young world out of the sky. Those Old Ones were gone now, inside the earth and under the sea; but their dead bodies had told their secrets in dreams to the first men, who formed a cult which had never died. This was that cult, and the prisoners said it had always existed and always would exist, hidden in distant wastes and dark places all over the world until the time when the great priest Cthulhu, from his dark house in the mighty city of R'lyeh under the waters, should rise and bring the earth again beneath his sway. Some day he would call, when the stars were ready, and the secret cult would always be waiting to liberate him.
Either one of the Great Old Ones or their alien priest, trapped under the Pacific in the corpse-city of R'lyeh.